Apparatus for filtering liquids

ABSTRACT

An apparatus for filtering liquids, such as fibre pulp suspensions, includes annular hollow filter discs each of which has two annular liquid pervious, opposed walls covered with filter material, spray nozzle means for cleaning the filter material, and means providing relative movement between the filter material and spray nozzle means. The annular opposed walls of each filter disc are only attached to each other at their radially outer ends, whereas they are spaced from each other at their radially inner ends, such that an annular opening is defined between the interior of the filter disc and a filtrate chamber extending centrally through the filter discs. The spray nozzle means extends from the filtrate chamber through the annular opening into each filter disc. As a result, an extremely pure fine fraction can be produced, clogging of the filtering material is efficiently counteracted, and the operative part of the filter material can be increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for filtering liquids,such as fibre pulp suspensions or polluted water.

2. Description of the Prior Art

A common type of filtering apparatuses for filtering liquids comprisesat least one annular hollow filter disc adapted to be at least partlyimmersed in a body of the liquid to be filtered and arrangedsubstantially transverse to an axis extending centrally through thefilter disc. The filter disc has two annular opposed walls, which haveradially inner ends and radially outer ends. At least one of the twoannular opposed walls is pervious by liquid and covered with filtermaterial. Wall means are connected to said opposed walls and form afiltrate chamber, which extends centrally through the hollow disc andwhich communicates with the interior of the latter. There is means forcreating a pressure difference between said body of liquid to befiltered and the interior of the hollow filter disc, such that a finefraction of the liquid is forced through the filter material into thehollow filter disc and passed therefrom into the filtrate chamber duringoperation. Fine fraction discharge means is provided for dischargingfine fraction from the filtrate chamber and spray nozzle means isprovided for spraying a cleansing fluid against the filter material forcleaning the latter. There is means for providing relative movementbetween the filter material and said spray nozzle means in thecircumferential direction of the filter disc.

Conventional filtering apparatuses of this type have their spray nozzlemeans arranged to spray the cleansing fluid in the form of high pressurejets of liquid from outside the filter disc. The filter disc isvertically positioned and partly immersed in the body of liquid to befiltered, so that part of the filter material is above the body ofliquid. The spray nozzle means is stationary and positioned above thebody of liquid, while the filter disc is rotatable about said axis.During operation, the spray nozzle means cleans the filter material fromparticles, as the filter material passes above the body of liquid. Thereason for having the spray nozzle means above the body of liquid is toprevent said high pressure jets of cleansing liquid from forcing courseparticles from the body of liquid into the filter holes of the filtermaterial. Otherwise, the filter material would be rapidly clogged andmade inoperative by such course particles during operation.

A drawback to such a conventional filtering apparatus is that some ofthe small particles which are adhered to the filter material duringoperation are pushed through the filter holes of the filter material bysaid high pressure jets of cleansing liquid, with the result that thepurity of the fine fraction is degraded. Also, it is inevitable thatsome particles of relatively course size adhered to the surface of thefilter material are pressed into the filter holes of the filter materialby said high pressure jets of cleansing liquid, so that, in the longrun, the filter material will be more or less clogged. This requiresregular stoppage of the conventional filtering apparatus in somefiltering applications, for cleaning clogged filter material bychemicals. Another drawback is that only about 60 to 65% of the filtermaterial is operative, i.e. the part of the filter material which isimmersed in the body of liquid during operation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a filtering apparatus,which enables production of a purer fine fraction, as compared withconventional filtering apparatuses.

Another object of the present invention is to provide a filteringapparatus, in which clogging of the filter material is efficientlycounteracted.

A further object of the present invention is to provide a filteringapparatus, in which the operative part of the filter material can beincreased, as compared with conventional filtering apparatuses.

These objects are fulfilled by means of an apparatus for filteringliquids, such as fibre pulp suspensions or polluted water, comprising:

at least one annular hollow filter disc adapted to be at least partlyimmersed in a body of a liquid to be filtered and arranged substantiallytransverse to an axis extending centrally through the filter disc, thedisc having two annular opposed walls, at least one of which is perviousto liquid and covered with filter material, the opposed walls havingradially inner and outer ends,

wall means connected to the annular opposed walls and forming a filtratechamber, which extends centrally through the hollow disc and whichcommunicates with the interior of the latter, the annular opposed wallsbeing solely attached to each other at their outer ends, whereas theyare spaced from each other at their inner ends, such that an annularopening is defined between the interior of the filter disc and thefiltrate chamber,

means for creating a pressure difference between the body of liquid tobe filtered and the interior of the hollow filter disc, such that a finefraction of the liquid is forced through the filter material into thehollow filter disc and passed therefrom into the filtrate chamber duringoperation,

fine fraction discharge means for discharging fine fraction from thefiltrate chamber,

spray nozzle means for spraying a cleansing fluid against the filtermaterial for cleaning the filter material, the spray nozzle meansextending from the filtrate chamber through the annular opening into theinterior of the filter disc, and

means for providing relative movement between the filter material andthe spray nozzle means in the circumferential direction of the filterdisc.

Since the spray nozzle means cleans the filter material from the finefraction side of the latter, the high pressure jets of cleansing fluidhit the filter material in a direction opposite to that of the flow offine fraction through the filter material. The effect of this is thatfine particles deposited in the filter holes of the filter material arepushed back to the outside of the filter disc, with the result that thecreated fine fraction of the liquid will be purer.

In addition to this, somewhat courser particles, which are adhered tothe outer surface of the filter material cannot be forced into the holesof the filter material and clog these by the high pressure jets ofcleansing fluid from the spray nozzle means. In some filteringapplications the course particles form mates of particles, which adhereto the filter material. Such mats of particles are readily removed fromthe filter material by said high pressure jets of fluid, such as air,since the jets of fluid operate from inside the filter material.

Thus, the filter material is most efficiently cleaned during operation.This has the positive consequence that a finer filter material, i.e. afilter material having smaller filter holes, can be utilized, ascompared with conventional filtering apparatuses. A finer filtermaterial used means the advantage that the purity of the fine fractionof the liquid is upgraded.

Since the spray means operates within the hollow disc, the latter may becompletely immersed in the body of liquid to be filtered. Thus,substantially 100% of the filter material can be operative, whichincreases the capacity of the filtering apparatus. This means that thecosts of a given production of filtered liquid, can be substantiallyreduced by means of the filtering apparatus of the invention, ascompared with the conventional filtering apparatuses.

Both of the body of liquid and the created fine fraction can besubjected to super-atmospheric pressures, if the filter disc iscompletely immersed in liquid, the pressure acting on the body of liquidbeing stronger than the pressure acting on the fine fraction. This meansthat the filtering apparatus of the invention can be integrated in apressurized process system. Since the filtering apparatus in such a casewould operate without admitting air to mix with the treated liquid, theadvantage is gained that the risk of reducing the capacity of subsequenttreating machinery, because of said mixing of air with the treatedliquid, is eliminated. This advantage is particularly important inconnection with treating fibre pulp suspensions.

In case the filter disc of the new filtering apparatus is completelyimmersed in the body of liquid to be filtered, the orientation of thefilter disc is optional. For instance, the filter disc may be verticallyor horizontally positioned in the body of the liquid, which makes thenew filtering apparatus adaptable to various kinds of installations. Thedesired pressure difference between the outside and the inside of thefilter material is readily adjusted by pump means adapted to pump finefraction from the filtrate chamber. As an alternative, or in combinationwith said pumping of fine fraction, pump means may be adapted to pumpthe liquid to be filtered into a closed chamber, in which the filterdisc is situated.

With the filter disc completely immersed in the body of liquid to befiltered, it is possible to arrange either the filter disc rotatableabout said axis, while the spray nozzle means is stationary, or viceversa. In case the filter disc is rotatable and the spray nozzle meansis stationary, it is suitable to position the filter disc vertically andadapt the fine fraction discharge means to discharge fine fraction fromthe filtrate chamber, such that a pool of fine fraction partly fillingthe filtrate chamber is created during operation. This enables the spraynozzle means to be arranged, such that at least the part of the filtermaterial which is above the pool of fine fraction is sprayed upon withcleansing fluid during operation. The consequence of this is that thepump energy consumption for pumping the cleansing fluid is reduced,since jets of cleansing fluid are passed through air instead of liquid.

The cleansing fluid can be in the form of liquid or gas, usually liquid.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-section of a first embodiment of thefiltering apparatus of the invention,

FIG. 2 is a vertical cross-section taken along line II--II of FIG. 1,

FIG. 3 is a horizontal cross-section taken along line III--III of FIG.2,

FIG. 4 is a part of a sectional view from above along line IV--IV ofFIG. 1,

FIG. 5 is a part of an enlarged sectional view of the filter disc shownin FIG. 1,

FIG. 6 is a modification of the embodiment shown in FIG. 5,

FIGS. 7 and 8 are sectional views along lines VII--VII and VIII--VIII,respectively, of FIG. 5,

FIG. 9 is a vertical cross-section of a second embodiment of thefiltering apparatus of the invention,

FIG. 10 is a longitudinal cross-section taken along line X--X of FIG. 9,

FIG. 11 is a part of a section taken along line XI--XI of FIG. 9,

FIG. 12 is a vertical cross-section of a third embodiment of thefiltering apparatus of the invention,

FIGS. 13 to 16 are parts of sectional views along lines XIII--XIII,XIV--XIV, XV--XV and XVI--XVI, respectively, of FIG. 12,

FIG. 17 is a part view of a modification of the embodiment shown i FIG.12,

FIG. 18 is a vertical cross-section of a fourth embodiment of theapparatus of the invention, showing one half of the apparatus,

FIG. 19 is a side view of a fifth embodiment of the apparatus of theinvention, and

FIG. 20 is a side view of a sixth embodiment of the apparatus of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The filtering apparatus shown in FIGS. 1 to 4 comprises a housing 1 withan inlet member 2 for the liquid to be filtered, and an outlet member 3at top of the housing 1 for discharging filtered liquid, i.e. createdcourse fraction. A pump 4 is provided to pump the liquid to be filteredinto the interior of the housing 1 via the inlet member 2. In thehousing 1 there are three annular hollow filter discs 5 spaced from eachother and arranged transverse to a horizontal axis, which extendscentrally through the filter discs 5. Each filter disc 5 comprises twoopposed side walls 6,7 having radially inner and outer circular ends.The side walls 6,7 of each filter disc 5 are attached to each other attheir outer ends via a peripheral wall 8. The walls 6 to 8 are madepervious by liquid and are covered with a suitable filter material, suchas cloth, felt, net or thin perforated plate.

The radially inner ends of the filter discs 5 are sealingly attached tocircular cylindrical wall portions 9, which form a hollow shaft 10, theinterior of which constitutes a filtrate chamber 11 for receiving acreated fine fraction. The hollow shaft 10 is journalled on thehousing 1. A drive motor 12 is engaged with a gable wall 13 at one endof the hollow shaft 10 for rotating the shaft 10. Opposite the gablewall 13, the hollow shaft has an open gable end, at which one of thecylindrical wall portions 9 is sealingly journalled on the housing 1. Afine fraction outlet pipe 14 is arranged on the housing 1 at the openend of the hollow shaft 10.

A stationary pipe 15 for cleansing liquid extends in the upper part ofthe filtrate chamber 11 and is journalled on the gable wall 13. From thepipe 15, three radial branch pipes 16 extend upwards into the threehollow discs 5, respectively. Each branch pipe 16 is provided with anumber of spray nozzles 17.

The side walls of the filter discs 5 are sealingly attached to the wallportions 9, via annular reinforcement members 18 having U-shapedcross-section (FIG. 5). Each filter disc 5 is provided with a pluralityof longitudinally U-shaped reinformcement members 19, which arecircumferentially spaced from each other and situated between the sidewalls 6,7. The walls 6,7 and 8 of each filter disc 5, are covered with afilter cloth 20, which constitutes the filter material. Each filtercloth 20 forms a circular bag, which has a centre hole for receiving oneof the cylindrical walls 9, and is clamped to the members 18 by means oftwo annular clamping members 21 having T-shaped cross-section. Each pairof adjacent annular members 18 and 21 are positioned to interlock witheach other, such that the filter cloth 20 is stretched between the twoopposed pairs of members 18 and 21, when the T-shaped clamping members21 are clamped to the respective U-shaped reinforcement members 18. Theperipheral wall 8 of each filter disc 5 is provided with a removable lid22, which faces an openable flap 23 in the filter cloth 20, to allowinspection and removal of the branch pipe 16 and the spray nozzles 17,for servicing of the latter. (FIGS. 7 and 8).

The modified filter discs 24 partly shown in FIG. 6 are formed by aplurality of circumferentially spaced beams 25, for instance twelve,which are bent in the radial direction of the filter discs 24 to formlongitudinally V-shaped support members for the side walls of therespective filter discs 24. The beams 25 are assembled by means ofcircumferentially extending bars 26, which are attached to the beams 25.Each side wall of the filter discs 24 is constituted by a flexiblethread member 27, which is spirally wound on the beams 25 substantiallyin the circumferential direction of the filter discs 24.

Filter cloths 28 of a design substantially similar to theabove-described filter cloth 20, enclose the side walls of therespective filter discs 24. The filter cloths 28 on adjacent filterdiscs 24 are joined to each other at their radially inner ends along anannular joint 29. By means of drawing-straps 30 extending along thejoints 29, the filter cloths 28 are stretched against the side walls ofthread members 27. The stationary pipe 15 for cleansing liquid isprovided with branch pipes 31, which have spray nozzles 32 adapted tothe V-shaped cross-section of the filter discs 24, such that the spraynozzles 32 are located at the same distance from the filter cloth 28.

At the top of the filter discs 5, there are inclined chute members 33extending substantially radially along the side walls 6, 7 for guidingcourse fraction up to the outlet member 3. A course fraction dischargepipe 34 is connected to the outlet member 3 via a pump 35. Between thepipe 34 and a mixing chamber 36 of the inlet member 2, there is afeed-back pipe 37, for feeding a part flow of the course fraction backinto the housing 1. Controllable valves 38 and 39 are provided in thefeed-back pipe 37 and the discharge pipe 34, respectively. The mixingchamber 36 communicates with the interior of the housing 1 through aplurality of distribution holes 40.

In operation, the hollow shaft 10 and the filter discs 5 are rotated bythe drive motor 12 in clockwise direction, as shown in FIG. 2. Theliquid to be filtered is pumped into the housing 1 via the inlet member2 by the pump 4. In the housing 1, a fine fraction of the liquid isforced through the filter material on the discs 5 into the interior ofthe discs 5. The fine fraction is collected in the filtrate chamber 11and discharged through the outlet 14 via an overflow member 14A, suchthat a liquid surface of the fine fraction is maintained in the filtratechamber 11.

Layers of course particles are built up on the outside of the filtermaterial of the discs 5. These layers are pushed off and disintegratedinto clumps of course particles by high pressure jets of cleansingliquid from the spray nozzles 17 (FIG. 4) and then guided by the chutemembers 33 to the outlet member 3. The created course fraction of theliquid is pumped from the outlet member 3 and discharged via the pipe 34and the valve 39.

A part flow of the created course fraction, the volume of which isdetermined by the valve 38, is fed back to the mixing chamber 36 via thefeed-back pipe 37. The part flow of created course fraction is added tothe liquid entering the mixing chamber 36, with the result that the rateof flow of the liquid flowing in the housing 1 from the inlet member 2to the outlet member 3 is increased. Said increased rate of flowimproves the entrainment of the clumps of course particles along thechute member 33 up to the outlet 3. Also, the course particleconcentration of the discharged course fraction is increased by saidadded part flow.

The filtering apparatus shown in FIGS. 9 to 11 comprises a container 41with an inlet 42 at one side of the container 41, for a liquid to befiltered, and an outlet 43 at the opposite side of the container 41, fora created course fraction of the liquid. Three filter discs 44 andcircular cylindrical wall portions 45, which are of identicalconstruction to the filter discs 5 and wall portions 9 (FIGS. 1 and 5),are assembled, such that the wall portions 45 form a hollow shaft 46,the interior of which constitutes a filtrate chamber 47. The hollowshaft 46 is of identical construction to the shaft 10 (FIG. 1), exceptthat the former has two open gable ends. At the two open gable ends, thehollow shaft 46 is sealingly journalled on the container 41. Two finefraction outlets 48 and 49 are arranged on the container 41 at therespective open ends of the hollow shaft 46. A drive motor 50 isarranged to rotate the hollow shaft 46 and the filter discs 44 via agear transmission 51 and 52.

Two stationary pipes 53 and 54 for fluid extend in the upper part of thefiltrate chamber 47. From the pipe 53, three radial branch pipes 55provided with spray nozzles 56 extend upward into the three hollow discs44. Likewise, from the pipe 54, three radial branch pipes 57 providedwith spray nozzles 58 extend upward into the hollow discs 44. The branchpipes 55 are somewhat circumferentially spaced from the branch pipes 57.

Entrainment members in the form of curved plates 59 extend from thecylindrical wall portions 45 along the side walls of the filter discs44, backward in the direction of rotation of the filter discs 44. Thecurved plates 59 are attached to the wall portions 45 via holder members60 (FIG. 11). The upper surfaces of the curved plates 59 are coated witha low-friction material, such as Teflon®. Stationary shower means 61 isarranged to flush the radially outer portions of the curved plates 59,as these pass by during rotation of the discs 44.

Four axially extending reinforcement beams 62 are attached to theradially outer ends of the filter discs 44, for stiffening the assemblyof filter discs 44 and cylindrical wall portions 45. The beams 62 alsooperate as entrainment means for entraining course particles orthickened slurry along the wall of the container 41 towards an overflowmember 63 at the outlet 43.

In operation, the liquid to be filtered, such as a fibre pulpsuspension, is supplied to the container 41 through the inlet 42. Thefilter discs 44 are rotated by the drive motor 50 in a direction, suchthat the parts of the filter discs 44 wich are located next to the inlet42 are displaced downwards, while the parts of the filter discs 44 whichare located next to the overflow member 63 are displaced upwards. Thehydrostatic pressure in the suspension in the container 41 causes a finefraction of the suspension, mainly containing water, to pass through thefilter material of the filter discs 44 into the filtrate chamber 47.From the filtrate chamber 47, the fine fraction passes over two overflowmembers 64 and 65, and is discharged via the outlet members 48 and 49.

Porous mats of fibres (course particles) are created on the parts of thefilter discs 44 which move in the suspension. As the mats on the filterdiscs 44 are displaced up above the suspension, they are pushed off thefilter material by high pressure jets of fluid (water or air) from thespray nozzles 58 and disintegrated into clumps of fibres. The jets ofthe spray nozzles 58 also perform a primary cleaning of the filtermaterial of the discs 44. A secondary cleaning of the filter material isperformed by jets of fluid (water or air) from the spray nozzles 56.

The thickened suspension containing said clumps of fibres is entrainedby the curved plates 59 and the beams 62 over the overflow member 63 tothe outlet 43. Clumps of fibres lifted above the suspension by thecurved plates 59 are flushed off the latter by the shower means 61, inthe position of the curved plates 59 illustrated in FIG. 9.

The filtering apparatus shown in FIGS. 12 to 17 is particularly suitedfor recovering fibres or other valuable particles from polluted water,and for cleaning water and other liquids. This apparatus comprises acontainer 66 with an inlet member 67 for the liquid to be filtered and acourse fraction discharge member 68 for a created course fraction of theliquid. A number of filter discs 69, which are of identical constructionto the filter discs 5 (FIGS. 1 and 5), are vertically arranged in thecontainer 66. The wall of the container 66 forms an overflow member 70at the course fraction discharge member 68. The overflow member 70extends inward along each side of the filter discs 69, such that chutemembers 71 are formed, which are inclined toward the discharge member 68(FIG. 16). The discharge member 68 is provided with a screw conveyor 72for discharging course fraction.

Each side of each filter discs 69 is provided with two outer entrainmentmembers 73 extending outward from the radially outer end of the filterdisc 69 and backwards in the direction of rotation of the filter disc69, and two inner entrainment members 74 extending outward from theradially inner end of the filter disc 69 and backward in the directionof rotation of the filter disc 69. The entrainment members 73 and 74 arepositioned, such that they just go clear of the overflow member 70, asthey pass the latter during rotation of the filter discs 69. Radiallybetween the outer and inner entrainment members 73 and 74 on each sideof the filter discs 69, there are two intermediate entrainment members75. The axial extension of the intermediate members 75 is limited, suchthat the members 75 just go clear of the overflow member 70, as theypass the latter during rotation of the filter discs 69 (FIG. 15).

Spray nozzle means 76 is arranged within each filter disc 69 andpositioned at the same level as the upper end of the overflow member 70(FIG. 13).

At the bottom of the container 66, there is a sluice member 77 with twovalves 78 and 79 below each filter disc 69, for discharging sedimentsfrom the interior of the housing 66 (FIG. 14).

A stationary wall member 80 extends axially and centrally through thefilter discs 69, and divides the filtrate chamber into three partchambers 81, 82 and 83.

In operation, the liquid to be filtered is supplied into the container66 via the inlet member 67. Heavy particles in the liquid sink bygravity and form sediments, which are entrained by the entrainmentmembers 73 to the sluice members 77. From these the sediments aredischarged at intervals by opening and closing the valves 78 and 79.

Three fractions of filtrate are forced into the hollow discs 69 anddistributed to the chamber 81 as a cloudy filtrate, to the chamber 82 asa clear filtrate and to the chamber 83 as an ultra-clear filtrate.Particles swimming in the liquid are entrained by the entrainmentmembers 73 to 75 up to the overflow member 70, where they are droppedinto the chute members 71 with the aid of the jets of fluid from thespray nozzle means 76.

In some filtering applications, it has been found that at least parts ofthe curved plates 59 (FIGS. 9 to 11) and the entrainment member 73-75(FIGS. 12 and 15) with advantage are made of liquid pervious material,such as net, grid or perforated plate. Additional shower means 84 may bearranged between the filter discs, to flush such perforated plates orentrainment members on their back sides, when the perforated plates orentrainment members are close to the overflow members during therotation of the filter discs (FIG. 17)

Brush members 85 may be arranged on the tips of the curved plates 59(FIGS. 9 to 11) or the entrainment members 73 to 75 (FIGS. 12 and 15),for brushing particles to the over-flow members 70 and 63 (FIG. 17).Said overflow members may be provided with axial elastic lists 86arranged to cover the gaps between the overflow members and the radiallyouter ends of the filter discs. As the curved plates 59 and theentrainment members 62 and 73 pass by during rotation of the discs, theelastic lists 86 are bent away by the former. The lists 86 prevent somesolid particles from falling back into the liquid, as the particles areflushed from the curved plate 59 or dropped from the entrainment members73 to 75.

In some filtering applications, it is desirable that no liquid or justan insignificant volume of liquid is discharged together with the solidparticles. This desire is fulfilled by the filtering apparatus shown inFIG. 18, which is substantially identical to the apparatus shown in FIG.9, except that the former has course fraction discharge means in theform of liquid pervious scoop members 87 arranged at the upper parts ofthe filter discs. The scoop members 87 are adapted to be pivoted atintervals between a first position, in which they receive courseparticles flushed away from the filter discs by spray nozzle means 88,and a second position (indicated in dash-dotted lines), in which saidreceived course particles are dropped from the scoop members 87 into anoutlet 89.

The filtering apparatus shown in FIG. 19 is immersed in a watercoursefor producing pre-cleaned water for various water consuming industry,such as heat exchanger plants, heat pump plants, processing plants andwater cleaning plants. The apparatus comprises three filter discs 90, ahollow shaft 91 and spray nozzles 92, which are of identicalconstruction to the filter discs 5, the hollow shaft 10 and the spraynozzles 17, respectively (FIG. 1). The hollow shaft 91 is journalled ona frame 93, which is situated on the bottom of the watercourse. Theframe 93 is provided with a drive motor 94 above the watercourse adaptedto rotate the discs 90 via a chain transmission.

A filtrate conduit 95 is connected to the interior of the hollow shaft91 via a chamber in the frame 93. There is a pump 96 in the filtrateconduit 95 for providing a pressure difference between the outside andthe inside of each filter disc 90. Said pressure difference is adjustedby means of a valve 97 in the filtrate conduit 95.

In the paper making industry, an assembly of filter discs 90 and spraynozzle means 92 (FIG. 19) may be utilized as a so-called pre-dewatererfor fibre pulp suspensions and be arranged together with a conventionalvacuum filter rotor in a common container, said assembly being at leastpartly immersed in the suspension.

The filtering apparatus shown in FIG. 20 is a modification of theapparatus shown in FIG. 19 and is particularly suited for deepwatercourses. The apparatus of FIG. 20 comprises three stationaryhorizontal filter discs 98 mounted on a vertical drum 99, the interiorof which constitutes a filtrate chamber. The undermost side wall of thefilter disc 98 is made of non-filter material and forms a funnel member100, the apex of which is connected to a filtrate conduit 101. There isa pump 102 in the filtrate conduit 101 for providing a pressuredifference between the outside and the inside of the filter discs 98.Said pressure difference is adjusted by means of a valve 103 in thefiltrate conduit 101.

A rotatable hollow shaft 104 for water extends from above thewatercourse into the drum 99 coaxially with the discs 98. The shaft 104is journalled on the drum 99 and is provided with three branch pipes 105extending into the respective discs 98. Each branch pipe 105 is providedwith a plurality of spray nozzles 106. Above the watercourse, there is adrive motor arrangement 107, for rotating the shaft 104 and the spraynozzles 106. The shaft 104 is connected to a stationary conduit 108 viaa sliding coupling 109. Purified water from the filtrate conduit 101 maybe supplied to the conduit 108 via a feed-back conduit 110 provided witha pump 111 (indicated in dash dotted lines). The apparatus is supportedon the bottom of the watercourse by means of legs 112 attached to thefunnel member 100.

In water purification plants, the filtering apparatus according to FIG.20 constitutes a low-price alternative to conventional equipment.

It should be appreciated that other combinations of the various featuresincluded in the embodiments of the invention described above areconcievable.

I claim:
 1. An apparatus for filtering liquids, such as fiber pulpsuspensions or polluted water, comprising:at least one annular hollowfilter disc adapted to be at least partly immersed in a body of a liquidto be filtered and arranged substantially transverse to an axisextending centrally through the filter disc, the disc having two annularopposed walls, at least one of which is pervious to liquid and coveredwith filter material, the opposed walls having radially inner and outerends; wall means connected to the annular opposed walls and forming afiltrate chamber, which extends centrally through the hollow disc andwhich communicates with the interior of the latter; the annular opposedwalls being solely attached to each other at their outer ends, whereasthey are spaced from each other at their inner ends, such that anannular opening is defined between the interior of the filter disc andthe filtrate chamber; means for creating a pressure difference betweenthe body of liquid to be filtered and the interior of the hollow filterdisc, such that a fine fraction of the liquid is forced through thefilter material into the hollow filter disc and passed therefrom intothe filtrate chamber during operation; fine fraction discharge means fordischarging fine fraction from the filtrate chamber; spray nozzle meansfor spraying a cleansing fluid against the filter material for cleaningthe filter material; the spray nozzle means extending from the filtratechamber through the annular opening into the interior of the filterdisc; and means for providing relative movement between the filtermaterial and the spray nozzle means in the circumferential direction ofthe filter disc.
 2. An apparatus according to claim 1, wherein thefilter disc is adapted to be completely immersed in the body of liquidto be filtered.
 3. An apparatus according to claim 2, wherein the spraynozzle means is stationary, and means is provided for rotating thefilter disc about the axis.
 4. An apparatus according to claim 3,further including a housing forming a closed chamber, in which thefilter disc is situated, the housing having inlet means for supplyingthe liquid to be filtered to the closed chamber, and course fractionoutlet means for discharging a created course fraction of the liquidfrom the closed chamber.
 5. An apparatus according to claim 4, whereinthe means for providing a pressure difference comprises pump meansadapted to pump the liquid to be filtered into the closed chamber viathe inlet means.
 6. An apparatus according to claim 5, wherein means isprovided for feeding part of the course fraction discharged from thecourse fraction outlet means to the inlet means, for increasing the rateof flow of the liquid flowing in the closed chamber between the inletmeans and the course fraction outlet means.
 7. An apparatus according toclaim 6, wherein the filter disc is positioned substantially verticallyin the body of liquid to be filtered, and the fine fraction dischargemeans is adapted to discharge fine fraction from the filtrate chamber,such that a pool of fine fraction partly filling the filtrate chamber iscreated during operation, the spray nozzle means being arranged to spraycleansing fluid against at least the part of the filter material whichis above the pool of fine fraction.
 8. An apparatus according to claim7, wherein the course fraction outlet means is arranged to dischargecourse fraction from the top of the closed chamber.
 9. An apparatusaccording to claim 5, wherein the filter disc is positionedsubstantially vertically in the body of liquid to be filtered, and thefine fraction discharge means is adapted to discharge fine fraction fromthe filtrate chamber, such that a pool of fine fraction partly fillingthe filtrate chamber is created during operation, the spray nozzle meansbeing arranged to spray cleansing fluid against at least the part of thefilter material which is above the pool of fine fraction.
 10. Anapparatus according to claim 9, wherein the course fraction outlet meansis arranged to discharge course fraction from the top of the closedchamber.
 11. An apparatus according to claim 2, wherein the filter discis stationary, and means is provided for rotating the spray nozzle meansabout the axis.
 12. An apparatus according to claim 11, wherein themeans for providing a pressure difference comprises pump means adaptedto pump fine fraction from the filtrate chamber.
 13. An apparatusaccording to claim 12, wherein the filter disc is positionedsubstantially horizontally in the body of liquid to be filtered.
 14. Anapparatus according to claim 13, wherein the filtrate chamber has abottom part located below the filter disc, and the fine fractiondischarge means comprises an outlet member arranged to discharge finefraction from the bottom part of the filtrate chamber.
 15. An apparatusaccording to claim 14, wherein the rotating means comprises a rotatablehollow shaft extending from above into the filtrate chamber coaxial withthe axis, and a drive motor for rotating the hollow shaft, the spraynozzle means being connected to the hollow shaft to be rotated therewithand to receive cleansing fluid through the hollow shaft.
 16. Anapparatus according to claim 1, wherein the spray nozzle means isstationary, and means is provided for rotating the filter disc about theaxis, the filter disc being positioned substantially vertically andpartly immersed in the body of liquid to be filtered, such that part ofthe filter material is above the body of liquid.
 17. An apparatusaccording to claim 16, further including a container containing the bodyof liquid to be filtered, inlet means for supplying the liquid to thecontainer, and course fraction discharge means for discharging a createdcourse fraction from the container, the spray nozzle means beingarranged to spray cleansing fluid against at least the part of thefilter material which is above the body of liquid.
 18. An apparatusaccording to claim 17, wherein each outer side of the filter disc isprovided with at least one entrainment member extending from the wallmeans backward in the rotational direction of the filter disc, forentraining created course fraction of the liquid in the containerradially outward and circumferentially forward to an overflow member inthe container.
 19. An apparatus according to claim 17, wherein thecourse fraction discharge means comprises a scoop member arranged ateach side of the filter disc, the scoop member being pivotable between afirst position, in which it receives created course fraction, when thelatter is loosened from the filter material by means of the spray nozzlemeans and a second position, in which the received course fraction isdropped from the scoop member to the outside of the container.
 20. Anapparatus according to claim 17, wherein the course fraction dischargemeans comprises a stationary chute member arranged at each side of thefilter disc, the chute member being inclined outward of the filter disc,for conducting course fraction dropped from the filter material out ofthe container.
 21. An apparatus for filtering liquids, such as fiberpulp suspensions or polluted water, comprising:at least two annularhollow filter discs adapted to be filtered and arranged substantiallytransverse to an axis extending centrally through the filter discs, eachdisc having two annular opposed walls, at least one of which is perviousby liquid and covered with filter material, the opposed walls havingradially inner and outer ends; wall means connected to the annularopposed walls and forming a filtrate chamber, which extends centrallythrough the hollow discs and which communicates with the interior of thelatter, the annular opposed walls of each filter disc being solelyattached to each other at their outer ends, whereas they are spaced fromeach other at their inner ends, such that an annular opening is definedbetween the interior of each filter disc and the filtrate chamber; meansfor creating a pressure difference between the body of liquid to befiltered and the interior of the hollow filter discs, such that a finefraction of the liquid is forced through the filter material into thehollow filter discs and passed therefrom into the filtrate chamberduring operation; fine fraction discharge means for discharging finefraction from the filtrate chamber; spray nozzle means for spraying acleansing fluid against the filter material for cleaning the filtermaterial, the spray nozzle means extending from the filtrate chamberthrough the annular opening of each filter disc into the interior ofeach filter disc; means for providing relative movement between thefilter material and spray nozzle means in the circumferential directionof the filter discs; and at least one axially extending reinforcementbeam attached to the walls of the filter discs at the outer ends of thefilter discs.